More image scanners that read an image of a document and electrically process the read image, or more printers that print an image, have employed a sheet feeding device that feeds a sheet such as a document from which an image is read or a paper sheet on which an image is printed to an image reading unit or a printing unit. Sheet feeding devices are known in which a bundle of documents mounted on a document mounting board are aligned with each other, a contact state of a document with a pickup roller is adjusted, or a speed at which a document moves to a sheet feed position is changed depending on a weight of the document so as to properly feed the document mounted on the document mounting board (for example, see Japanese Laid-open Patent Publication No. 2009-242102, Japanese Laid-open Patent Publication No. 2007-137526, and Japanese Laid-open Patent Publication No. 2008-30939.)
However, in a sheet feeding device in which an document mounted on a document mounting board moves to a position of a pickup roller by its own weight, when the number of documents is large and the weight of the documents is large, a separation member that separates the documents other than the document fed by the pickup roller may be pushed up by the bundle of documents. In this case, it may be difficult to properly separate the documents with the separation member and so-called overlap feeding of feeding a plurality of documents may occur. The overlap feeding causes a paper jam and thus does not need to occur. However, by only aligning a bundle of documents or adjusting a contact state of the document with the pickup roller, it is difficult to suppress occurrence of the overlap feeding and it is very difficult to prevent from occurring the overlap feeding.